This application claims priority from Korean Patent Application No. 10-2004-0048934, filed on Jun. 28, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
Apparatuses consistent with the present invention relate to measuring temperature using a change of magnetic field, and more particularly, to measuring temperature of a wafer in real time using a change of magnetic field at a wafer thermal processing system.
2. Description of the Related Art
Photolithography is a process used to pattern semiconductor materials stacked on a wafer. During photolithography, a thermal process is performed three times. A first thermal process is a soft bake which is performed after coating a photoresist on an entire surface of the wafer. The soft bake is performed so as to maintain uniformity of the photoresist coated on the wafer. The soft bake removes solvent from the photoresist by heating the wafer on a hot plate at 80-110° C. After the soft bake, the wafer is exposed to ultraviolet rays using a projection exposure system or the like and predetermined patterns are formed on the photoresist. After the exposure, a post exposure bake (PEB) is performed as a second thermal process. The PEB is performed so as to prevent unnecessary exposure at an exposure boundary due to scattering of ultraviolet rays. In the PEB, the wafer is baked on a hot plate at about 150° C. After the exposure, developer solution is sprayed onto the wafer, and a region onto which light is incident, or a region onto which no light is incident, is removed due to a chemical reaction. After development, a hard bake is performed as a third thermal process. The hard bake removes solvent from the photoresist and hardens the photoresist by heating the wafer on a hot plate at about 150° C.
In these thermal processes, temperature distribution and temperature uniformity across the wafer are important processing variables which have the greatest influence on quality and dispersion of line width in a semiconductor circuit. Even a slight error in temperature changes solubility of the photoresist. As a result, line widths of patterns become different. In order to obtain good quality, temperature variation must be restricted to ±0.1° C. over the entire wafer heated on the hot plate. Therefore, it is necessary to measure the wafer temperature during the thermal process to determine whether the wafer is being heated uniformly.
Conventional methods for measuring wafer temperature include a contact temperature measuring method and a non-contact temperature measuring method.
According to the contact temperature measuring method, before the thermal process is actually performed on the wafer, whether the wafer is uniformly heated is examined by heating a sensor wafer on the hot plate. FIGS. 1A and 1B are views of the sensor wafer. Referring to FIG. 1A, a plurality of temperature sensors 120 are installed on a sensor wafer 110 and a change of temperature on the sensor wafer 110 is measured through wires 125 connected to the temperature sensors 120. Referring to FIG. 1B, instead of connecting the wires complexly, a plurality of temperature sensors 120 and a memory 130 are installed on the sensor wafer 110 and a change of temperature on the sensor wafer 110 is written to the memory 130. The contact temperature sensor used herein includes a resistance thermal detector (RTD) which detects electrical resistance, a thermocouple (TC) which detects thermo-electromotive force, a metal or glass thermometer which detects thermal expansion, an integrated circuit (IC) temperature sensor which operates based on a temperature characteristic of a silicon transistor, and so on. Through these methods, a heating condition of the hot plate is adjusted until the temperatures measured by the temperature sensors become uniform. After it is checked that the entire area of the sensor wafer is uniformly heated, the wafer is actually loaded and the thermal process is performed thereon.
According to this conventional contact temperature measuring method, the temperature sensors must directly contact the wafer. However, it is difficult to correctly contact the temperature sensors with the object to be measured. Also, the conventional contact temperature measuring method is used to set an appropriate heating condition using the sensor wafer in advance, not while the thermal process is actually being performed. Therefore, it is impossible to measure the actual wafer temperature while the thermal process is being performed. That is, the conventional method does not facilitate measurement of the wafer temperature in real time during the thermal process. When an external environment changes during the thermal process, it is impossible to promptly cope with the change. As a result, the proportion of defective areas may increase during the thermal process. Also, when a defect occurs during a complex semiconductor manufacturing process, it is impossible to examine and reliably determine which process caused the defect. In order to make such a determination, the process must be stopped to measure the temperature using the sensor wafer. Thus, the semiconductor manufacturing process is delayed and manufacturing cost is increased.
In order to solve these problems, a non-contact temperature measuring method has been proposed. The non-contact temperature measuring method measures the wafer temperature in real time during the thermal process and obtains temperature history information which contains a change of temperature with respect to time during the thermal process. The non-contact temperature measuring method makes use of a principle of black body radiation. That is, the temperature is measured by detecting infrared rays radiated from the heated object. FIG. 2 is a sectional view illustrating the non-contact temperature measuring method. Referring to FIG. 2, while a wafer 110 is heated on a hot plate 140, a temperature of the wafer 110 is measured in real time using a plurality of temperature sensors 160 which are installed below a cap 150 of a thermal processing system. An infrared thermometer is widely used as the temperature sensor 160. The infrared thermometer includes a quantum infrared sensor which measures an internal photoelectric effect of semiconductor material, a thermal infrared sensor which measures change in a material constant with respect to temperature, and so on. Also, the thermal infrared sensor includes a pyroelectric infrared sensor which operates based on the pyroelectric effect in which electricity is generated in ferroelectric material bombarded by infrared radiation.
However, the conventional non-contact temperature measuring method has problems in that radiation generated from the object to be measured must sufficiently reach the sensor and an effective emissivity of the object to be measured must be accurately known, or its reproduction must be possible. Also, the infrared sensor used in the non-contact temperature measuring method has a relatively high accuracy at high temperatures (generally, higher than 1000° C.), but has a relatively low accuracy at the low temperatures (hundreds of ° C.) at which the thermal process is preformed. Further, as described above, temperature variation must within ±0.1° C. over the entire wafer. However, since resolution of the infrared sensor is about 1° C., use of the infrared sensor is inappropriate during the thermal process.